H LV Switch Gear Functions and Selection

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Schneider Electric - Electrical installation guide 2010

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S c h n e i d e r E l e c t r i c - a l l r i g h t s r e s e r v e d

Chapter H

LV switchgear: unctions &

selection

Contents

The basic unctions o LV switchgear H2

1.1 Electrical protection H2

1.2 Isolation H3

1.3 Switchgear control H4

The switchgear H5

2.1 Elementary switching devices H5

2.2 Combined switchgear elements H9

Choice o switchgear H0

3.1 Tabulated unctional capabilities H10

3.2 Switchgear selection H10

Circuit-breaker H

4.1 Standards and description H11

4.2 Fundamental characteristics o a circuit-breaker H13

4.3 Other characteristics o a circuit-breaker H15

4.4 Selection o a circuit-breaker H18

4.5 Coordination between circuit-breakers H22

4.6 Discrimination MV/LV in a consumer’s substation H28

2

3

4

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.2 Isolation

The aim o isolation is to separate a circuit or apparatus (such as a motor, etc.) romthe remainder o a system which is energized, in order that personnel may carry outwork on the isolated part in perect saety.

In principle, all circuits o an LV installation shall have means to be isolated.In practice, in order to maintain an optimum continuity o service, it is preerred toprovide a means o isolation at the origin o each circuit.

An isolating device must ull the ollowing requirements:

b All poles o a circuit, including the neutral (except where the neutral is a PENconductor) must open(1)

b It must be provided with a locking system in open position with a key (e.g. bymeans o a padlock) in order to avoid an unauthorized reclosure by inadvertence

b It must comply with a recognized national or international standard(e.g. IEC 60947-3) concerning clearance between contacts, creepage distances,overvoltage withstand capability, etc.:

Other requirements apply:v

Verication that the contacts o the isolating device are, in act, open.The verication may be:

- Either visual, where the device is suitably designed to allow the contacts to be seen(some national standards impose this condition or an isolating device located at theorigin o a LV installation supplied directly rom a MV/LV transormer)

- Or mechanical, by means o an indicator solidly welded to the operating shato the device. In this case the construction o the device must be such that, in theeventuality that the contacts become welded together in the closed position, theindicator cannot possibly indicate that it is in the open position

v Leakage currents. With the isolating device open, leakage currents between theopen contacts o each phase must not exceed:

- 0.5 mA or a new device- 6.0 mA at the end o its useul liev Voltage-surge withstand capability, across open contacts. The isolating device,when open must withstand a 1.2/50 μs impulse, having a peak value o 6, 8 or 12 kVaccording to its service voltage, as shown in Figure H2. The device must satisythese conditions or altitudes up to 2,000 metres. Correction actors are given in

IEC 60664-1 or altitudes greater than 2,000 metres.Consequently, i tests are carried out at sea level, the test values must be increasedby 23% to take into account the eect o altitude. See standard IEC 60947.

The basic unctions o

LV switchgear

A state o isolation clearly indicated by an

approved “ail-proo” indicator, or the visible

separation o contacts, are both deemed to satisy the national standards o many countries

(1) the concurrent opening o all live conductors, while not

always obligatory, is however, strongly recommended (orreasons o greater saety and acility o operation). The neutral

contact opens ater the phase contacts, and closes beorethem (IEC 60947-1).

Service (nominal Impulse withstandvoltage peak voltage category

(V) (or 2,000 metres)(kV)

III IV

230/400 4 6

400/690 6 8

690/1,000 8 12

Fig. H2 : Peak value o impulse voltage according to normal service voltage o test specimen.The degrees III and IV are degrees o pollution defned in IEC 60664-1

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2 The switchgear

2. Elementary switching devices

Disconnector (or isolator) (see Fig. H5)

This switch is a manually-operated, lockable, two-position device (open/closed)which provides sae isolation o a circuit when locked in the open position. Itscharacteristics are dened in IEC 60947-3. A disconnector is not designed to makeor to break current(1) and no rated values or these unctions are given in standards.It must, however, be capable o withstanding the passage o short-circuit currentsand is assigned a rated short-time withstand capability, generally or 1 second,unless otherwise agreed between user and manuacturer. This capability is normallymore than adequate or longer periods o (lower-valued) operational overcurrents,such as those o motor-starting. Standardized mechanical-endurance, overvoltage,and leakage-current tests, must also be satised.

Load-breaking switch (see Fig. H6)

This control switch is generally operated manually (but is sometimes provided withelectrical tripping or operator convenience) and is a non-automatic two-positiondevice (open/closed).

It is used to close and open loaded circuits under normal unaulted circuit conditions.

It does not consequently, provide any protection or the circuit it controls.

IEC standard 60947-3 denes:

b The requency o switch operation (600 close/open cycles per hour maximum)

b Mechanical and electrical endurance (generally less than that o a contactor)

b Current making and breaking ratings or normal and inrequent situations

When closing a switch to energize a circuit there is always the possibility thatan unsuspected short-circuit exists on the circuit. For this reason, load-breakswitches are assigned a ault-current making rating, i.e. successul closure againstthe electrodynamic orces o short-circuit current is assured. Such switches arecommonly reerred to as “ault-make load-break” switches. Upstream protectivedevices are relied upon to clear the short-circuit ault

Category AC-23 includes occasional switching o individual motors. The switchingo capacitors or o tungsten lament lamps shall be subject to agreement betweenmanuacturer and user.

The utilization categories reerred to in Figure H7 do not apply to an equipmentnormally used to star t, accelerate and/or stop individual motors.

Example

A 100 A load-break switch o category AC-23 (inductive load) must be able:

b To make a current o 10 In (= 1,000 A) at a power actor o 0.35 lagging

b To break a current o 8 In (= 800 A) at a power actor o 0.45 lagging

b To withstand short duration short-circuit currents when closed

(1) i.e. a LV disconnector is essentially a dead system

switching device to be operated with no voltage on either sideo it, particularly when closing, because o the possibility o an

unsuspected short-circuit on the downstream side. Interlockingwith an upstream switch or circuit-breaker is requently used.

Fig. H7 : Utilization categories o LV AC switches according to IEC 60947-3

Fig. H5 : Symbol or a disconnector (or isolator)

Fig. H6 : Symbol or a load-break switch

Utilizat ion category Typical applications Cos ϕ Making Breaking

Frequent Inrequent current x In current x In

operations operations

AC-20A AC-20B Connecting and disconnecting - - -under no-load conditions

AC-21A AC-21B Switching o resistive loads 0.95 1.5 1.5including moderate overloads

AC-22A AC-22B Switching o mixed resistive 0.65 3 3and inductive loads, includingmoderate overloads

AC-23A AC-23B Switching o motor loads or 0.45 or I y 100 A 10 8

other highly inductive loads 0.35 or I > 100 A

H - LV switchgear: unctions & selection

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Remote control switch (see Fig. H8)

This device is extensively used in the control o lighting circuits where the depression

o a pushbutton (at a remote control position) will open an already-closed switch orclose an opened switch in a bistable sequence.

Typical applications are:

b Two-way switching on stairways o large buildings

b Stage-lighting schemes

b Factory illumination, etc.

Auxiliary devices are available to provide:

b Remote indication o its state at any instant

b Time-delay unctions

b Maintained-contact eatures

Contactor (see Fig. H9)

The contactor is a solenoid-operated switching device which is generally heldclosed by (a reduced) current through the closing solenoid (although variousmechanically-latched types exist or specic duties). Contactors are designed to

carry out numerous close/open cycles and are commonly controlled remotely byon-o pushbuttons. The large number o repetitive operating cycles is standardized intable VIII o IEC 60947-4-1 by:

b The operating duration: 8 hours; uninterrupted; intermittent; temporary o 3, 10, 30,60 and 90 minutes

b Utilization category: or example, a contactor o category AC3 can be used or thestarting and stopping o a cage motor

b The start-stop cycles (1 to 1,200 cyles per hour)

b Mechanical endurance (number o o-load manœuvres)

b Electrical endurance (number o on-load manœuvres)

b A rated current making and breaking perormance according to the category outilization concerned

Example:

A 150 A contactor o category AC3 must have a minimum current-breaking capabilityo 8 In (= 1,200 A) and a minimum current-making rating o 10 In (= 1,500 A) at a

power actor (lagging) o 0.35.

Discontactor()

A contactor equipped with a thermal-type relay or protection against overloadingdenes a “discontactor”. Discontactors are used extensively or remote push-buttoncontrol o lighting circuits, etc., and may also be considered as an essential elementin a motor controller, as noted in sub-clause 2.2. “combined switchgear elements”.The discontactor is not the equivalent o a circuit-breaker, since its short-circuitcurrent breaking capability is limited to 8 or 10 In. For short-circuit protectionthereore, it is necessary to include either uses or a circuit-breaker in series with,and upstream o, the discontactor contacts.

Fuses (see Fig. H0)

The rst letter indicates the breaking range:b “g” use-links (ull-range breaking-capacity use-link)

b “a” use-links (partial-range breaking-capacity use-link)

The second letter indicates the utilization category; this letter denes with accuracythe time-current characteristics, conventional times and currents, gates.

For exampleb “gG” indicates use-links with a ull-range breaking capacity or general application

b “gM” indicates use-links with a ull-range breaking capacity or the protection omotor circuitsb “aM” indicates use-links with a partial range breaking capacity or the protection omotor circuits

Fuses exist with and without “use-blown” mechanical indicators. Fuses break acircuit by controlled melting o the use element when a current exceeds a givenvalue or a corresponding period o time; the current/time relationship beingpresented in the orm o a perormance curve or each type o use. Standards denetwo classes o use:b Those intended or domestic installations, manuactured in the orm o a cartridgeor rated currents up to 100 A and designated type gG in IEC 60269-1 and 3

b

Those or industrial use, with cartridge types designated gG (general use); and gMand aM (or motor-circuits) in IEC 60269-1 and 2

Fig. H8 : Symbol or a bistable remote control switch

Control

circuit

Power

circuit

Fig. H9 : Symbol or a contactor

(1) This term is not dened in IEC publications but is commonlyused in some countries.

Two classes o LV cartridge use are very widely used:

b For domestic and similar installations type gG

b For industrial installations type gG, gM or aM

Fig. H10 : Symbol or uses

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The main dierences between domestic and industrial uses are the nominalvoltage and current levels (which require much larger physical dimensions) and

their ault-current breaking capabilities. Type gG use-links are oten used or theprotection o motor circuits, which is possible when their characteristics are capableo withstanding the motor-starting current without deterioration.

A more recent development has been the adoption by the IEC o a use-type gM ormotor protection, designed to cover starting, and short-circuit conditions. This type ouse is more popular in some countries than in others, but at the present time theaM use in combination with a thermal overload relay is more-widely used.A gM use-link, which has a dual rating is characterized by two current values. Therst value In denotes both the rated current o the use-link and the rated current othe useholder; the second value Ich denotes the time-current characteristic o theuse-link as dened by the gates in Tables II, III and VI o IEC 60269-1.

These two ratings are separated by a letter which denes the applications.

For example: In M Ich denotes a use intended to be used or protection omotor circuits and having the characteristic G. The rst value In corresponds tothe maximum continuous current or the whole use and the second value Ichcorresponds to the G characteristic o the use link. For urther details see note at the

end o sub-clause 2.1.

An aM use-link is characterized by one current value In and time-currentcharacteristic as shown in Figure H14 next page.

Important: Some national standards use a gI (industrial) type use, similar in all mainessentails to type gG uses.Type gI uses should never be used, however, in domestic and similar installations.

Fusing zones - conventional currents

The conditions o using (melting) o a use are dened by standards, according totheir class.

Class gG uses

These uses provide protection against overloads and short-circuits.Conventional non-using and using currents are standardized, as shown inFigure H2 and in Figure H3.

b The conventional non-using current In is the value o current that the usible

element can carry or a specied time without melting.Example: A 32 A use carrying a current o 1.25 In (i.e. 40 A) must not melt in lessthan one hour (table H13)

b The conventional using current I (= I2 in Fig. H12) is the value o current whichwill cause melting o the usible element beore the expiration o the specied time.

Example: A 32 A use carrying a current o 1.6 In (i.e. 52.1 A) must melt in one houror less

IEC 60269-1 standardized tests require that a use-operating characteristic liesbetween the two limiting curves (shown in Figure H12) or the particular use undertest. This means that two uses which satisy the test can have signicantly dierentoperating times at low levels o overloading.

gM fuses require a separate overload relay, as described in the note at the end of sub-clause 2.1.

1 hour

t

Minimum

pre-arcing

time curve

Fuse-blow

curve

I

Inf I2

2 The switchgear

Fig. H12 : Zones o using and non-using or gG and gM uses

(1) Ich or gM uses

Fig. H13 : Zones o using and non-using or LV types gG and gM class uses (IEC 60269-1

and 60269-2-1)

Rated current() Conventional non- Conventional Conventional

In (A) using current using current time (h)

In I2

In y 4 A 1.5 In 2.1 In 1

4 < In < 16 A 1.5 In 1.9 In 1

16 < In y 63 A 1.25 In 1.6 In 1

63 < In y 160 A 1.25 In 1.6 In 2

160 < In y 400 A 1.25 In 1.6 In 3

400 < In 1.25 In 1.6 In 4

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b The two examples given above or a 32 A use, together with the oregoing noteson standard test requirements, explain why these uses have a poor perormance in

the low overload rangeb It is thereore necessary to install a cable larger in ampacity than that normallyrequired or a circuit, in order to avoid the consequences o possible long termoverloading (60% overload or up to one hour in the worst case)

By way o comparison, a circuit-breaker o similar current rating:

b Which passes 1.05 In must not trip in less than one hour; and

b When passing 1.25 In it must trip in one hour, or less (25% overload or up to onehour in the worst case)

Class aM (motor) uses

These uses aord protection against short-circuit currents only and must necessarilybe associated with other switchgear (such as discontactors or circuit-breakers) inorder to ensure overload protection < 4 In. They are not thereore autonomous. SinceaM uses are not intended to protect against low values o overload current, no levelso conventional non-using and using currents are xed. The characteristic curves ortesting these uses are given or values o ault current exceeding approximately 4 In(see Fig. H4), and uses tested to IEC 60269 must give operating curves which allwithin the shaded area.

Note: the small “arrowheads” in the diagram indicate the current/time “gate” valuesor the dierent uses to be tested (IEC 60269).

Rated short-circuit breaking currents

A characteristic o modern cartridge uses is that, owing to the rapidity o usionin the case o high short-circuit current levels(1), a current cut-o begins beorethe occurrence o the rst major peak, so that the ault current never reaches itsprospective peak value (see Fig. H5).

This limitation o current reduces signicantly the thermal and dynamic stresseswhich would otherwise occur, thereby minimizing danger and damage at the aultposition. The rated short-circuit breaking current o the use is thereore based on therms value o the AC component o the prospective ault current.

No short-circuit current-making rating is assigned to uses.

ReminderShort-circuit currents initially contain DC components, the magnitude and duration owhich depend on the XL /R ratio o the ault current loop.

Close to the source (MV/LV transormer) the relationship Ipeak / Irms (oAC component) immediately ollowing the instant o ault, can be as high as 2.5(standardized by IEC, and shown in Figure H6 next page).

At lower levels o distribution in an installation, as previously noted, XL is smallcompared with R and so or fnal circuits Ipeak / Irms ~ 1.41, a condition whichcorresponds with Figure H15.

The peak-current-limitation eect occurs only when the prospective rmsAC component o ault current attains a certain level. For example, in the Figure H16graph, the 100 A use will begin to cut o the peak at a prospective ault current(rms) o 2 kA (a). The same use or a condition o 20 kA rms prospective currentwill limit the peak current to 10 kA (b). Without a current-limiting use the peakcurrent could attain 50 kA (c) in this particular case. As already mentioned, at lowerdistribution levels in an installation, R greatly predominates XL, and ault levels are

generally low. This means that the level o ault current may not attain values highenough to cause peak current limitation. On the other hand, the DC transients (in thiscase) have an insignicant eect on the magnitude o the current peak, as previouslymentioned.

Note: On gM use ratingsA gM type use is essentially a gG use, the usible element o which corresponds tothe current value Ich (ch = characteristic) which may be, or example, 63 A. This isthe IEC testing value, so that its time/ current characteristic is identical to that o a63 A gG use.This value (63 A) is selected to withstand the high starting currents o a motor, thesteady state operating current (In) o which may be in the 10-20 A range.

This means that a physically smaller use barrel and metallic parts can be used,since the heat dissipation required in normal service is related to the lower gures(10-20 A). A standard gM use, suitable or this situation would be designated 32M63(i.e. In M Ich).

The rst current rating In concerns the steady-load thermal perormance o the

uselink, while the second current rating (Ich) relates to its (short-time) starting-current perormance. It is evident that, although suitable or short-circuit protection,

Class aM uses protect against short-circuit currents only, and must always be associated with another device which protects against

overload

(1) For currents exceeding a certain level, depending on theuse nominal current rating, as shown below in Figure H16.

x In

t

4In

Fuse-blown

cur ve

Minimum

pre-arcing

time cur ve

Fig. H14 : Standardized zones o using or type aM uses (all

current ratings)

I

0.005 s

0.02 s

0.01 s t

Prospective

fault-current peak

rms value of the AC

component of theprospective fault curent

Current peaklimited by the fuse

Tf TaTtc

Tf: Fuse pre-arc fusing timeTa: Arcing time

Ttc: Total fault-clearance time

Fig. H15 : Current limitation by a use

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overload protection or the motor is not provided by the use, and so a separatethermal-type relay is always necessary when using gM uses. The only advantage

oered by gM uses, thereore, when compared with aM uses, are reduced physicaldimensions and slightly lower cost.

2.2 Combined switchgear elements

Single units o switchgear do not, in general, ull all the requirements o the threebasic unctions, viz: Protection, control and isolation.

Where the installation o a circuit-breaker is not appropriate (notably where theswitching rate is high, over extended periods) combinations o units specicallydesigned or such a perormance are employed. The most commonly-usedcombinations are described below.

Switch and use combinations

Two cases are distinguished:

b The type in which the operation o one (or more) use(s) causes the switch to open.This is achieved by the use o uses tted with striker pins, and a system o switchtripping springs and toggle mechanisms (see Fig. H7)

b The type in which a non-automatic switch is associated with a set o uses in acommon enclosure.In some countries, and in IEC 60947-3, the terms “switch-use” and “use-switch”have specic meanings, viz:v A switch-use comprises a switch (generally 2 breaks per pole) on the upstreamside o three fxed use-bases, into which the use carriers are inserted (see Fig. H8)

v A use-switch consists o three switch blades each constituting a double-break perphase.

These blades are not continuous throughout their length, but each has a gap in thecentre which is bridged by the use cartridge. Some designs have only a single breakper phase, as shown in Figure H9.

Fig. H17 : Symbol or an automatic tripping switch-use

Fig. H19 : Symbol or a non-automatic switch-use

Fig. H18 : Symbol or a non-automatic use-switch

2 The switchgear

Fig. H16 : Limited peak current versus prospective rms values o the AC component o ault current or LV uses

1 2 5 10 20 50 1001

2

5

10

20

50

100

(a)

(b)

(c)

Peak current

cut-off

characteristic

curves

Maximum possible currentpeak characteristic

i.e. 2.5 Irms (IEC)

160A

100A

50A

Nominal

fuse

ratings

Prospective fault

current (kA) peak

AC component of prospective

fault current (kA) rms

Fig. H20 : Symbol or a use disconnector + discontactor

Fig. H21 : Symbol for a fuse-switch disconnector + discontactor

The current range or these devices is limited to 100 A maximum at 400 V 3-phase,

while their principal use is in domestic and similar installations. To avoid conusion

between the rst group (i.e. automatic tripping) and the second group, the term

“switch-use” should be qualied by the adjectives “automatic” or “non-automatic”.

Fuse – disconnector + discontactor

Fuse - switch-disconnector + discontactor

As previously mentioned, a discontactor does not provide protection against short-

circuit aults. It is necessary, thereore, to add uses (generally o type aM) to perorm

this unction. The combination is used mainly or motor control circuits, where the

disconnector or switch-disconnector allows sae operations such as:

b The changing o use links (with the circuit isolated)

b Work on the circuit downstream o the discontactor (risk o remote closure o the

discontactor)

The use-disconnector must be interlocked with the discontactor such that no opening

or closing manœuvre o the use disconnector is possible unless the discontactor is

open ( Figure H20), since the use disconnector has no load-switching capability.

A use-switch-disconnector (evidently) requires no interlocking (Figure H2).

The switch must be o class AC22 or AC23 i the circuit supplies a motor.

Circuit-breaker + contactor

Circuit-breaker + discontactor

These combinations are used in remotely controlled distribution systems in which the

rate o switching is high, or or control and protection o a circuit supplying motors.

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3 Choice o switchgear

3. Tabulated unctional capabilities

Ater having studied the basic unctions o LV switchgear (clause 1, Figure H1) andthe dierent components o switchgear (clause 2), Figure H22 summarizes thecapabilities o the various components to perorm the basic unctions.

(1) Where cut-o o all active conductors is provided

(2) It may be necessary to maintain supply to a braking system

(3) I it is associated with a thermal relay (the combination is commonly reerred to as a “discontactor”)

(4) In certain countries a disconnector with visible contacts is mandatory at the origin o a LV installation supplied directly rom a MV/LV transormer

(5) Certain items o switchgear are suitable or isolation duties (e.g. RCCBs according to IEC 61008) without being explicitly marked as such

3.2 Switchgear selection

Sotware is being used more and more in the eld o optimal selection o switchgear.

Each circuit is considered one at a time, and a list is drawn up o the requiredprotection unctions and exploitation o the installation, among those mentioned inFigure H22 and summarized in Figure H1.

A number o switchgear combinations are studied and compared with each otheragainst relevant criteria, with the aim o achieving:

b Satisactory perormance

b Compatibility among the individual items; rom the rated current In to the ault-levelrating Icu

b Compatibility with upstream switchgear or taking into account its contribution

b Conormity with all regulations and specications concerning sae and reliablecircuit perormance

In order to determine the number o poles or an item o switchgear, reerence ismade to chapter G, clause 7 Fig. G64. Multiunction switchgear, initially more costly,reduces installation costs and problems o installation or exploitation. It is oten oundthat such switchgear provides the best solution.

Isolation Control Electrical protection

Switchgear Functional Emergency Emergency Switching or Overload Short-circuit Electric

item switching stop mechanical shock (mechanical) maintenance

Isolator (or b disconnector)(4)

Switch(5) b b b (1) b (1) (2) b

Residual b b b (1) b (1) (2) b b device

(RCCB)(5)

Switch- b b b (1) b (1) (2) b disconnector

Contactor b b (1) b (1) (2) b b (3)

Remote control b b (1) b

switch

Fuse b b b

Circuit b b (1) b (1) (2) b b b

breaker

Circuit-breaker b b b (1) b (1) (2) b b b disconnector(5)

Residual b b b (1) b (1) (2) b b b b and overcurrent

circuit-breaker(RCBO)(5)

Point o Origin o each All points where, In general at the At the supply At the supply Origin o each Origin o each Origin o circuitsinstallation circuit or operational incoming circuit point to each point to each circuit circuit where the(general reasons it may to every machine machine earthing system

principle) be necessary distribution and/or on the is appropriateto stop the board machine TN-S, IT, TT

process concerned

Fig. H22 : Functions ulflled by dierent items o switchgear

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4 Circuit-breaker

The circuit-breaker/disconnector ulflls all o the basic switchgear unctions, while, by means o accessories, numerous other possibilities exist

As shown in Figure H23 the circuit-breaker/ disconnector is the only item oswitchgear capable o simultaneously satisying all the basic unctions necessary in

an electrical installation.Moreover, it can, by means o auxiliary units, provide a wide range o other unctions,or example: indication (on-o - tripped on ault); undervoltage tripping; remotecontrol… etc. These eatures make a circuit-breaker/ disconnector the basic unit oswitchgear or any electrical installation.

Fig. H23 : Functions perormed by a circuit-breaker/disconnector

4. Standards and description

Standards

For industrial LV installations the relevant IEC standards are, or are due to be:

b 60947-1: general rulesb 60947-2: part 2: circuit-breakers

b 60947-3: part 3: switches, disconnectors, switch-disconnectors and usecombination units

b 60947-4: part 4: contactors and motor star ters

b 60947-5: part 5: control-circuit devices and switching elements

b 60947-6: part 6: multiple unction switching devices

b 60947-7: part 7: ancillary equipment

For domestic and similar LV installations, the appropriate standard is IEC 60898, oran equivalent national standard.

Description

Figure H24 shows schematically the main parts o a LV circuit-breaker and its ouressential unctions:

b The circuit-breaking components, comprising the xed and moving contacts and

the arc-dividing chamberb The latching mechanism which becomes unlatched by the tripping device ondetection o abnormal current conditions

This mechanism is also linked to the operation handle o the breaker.

b A trip-mechanism actuating device:

v Either: a thermal-magnetic device, in which a thermally-operated bi-metal stripdetects an overload condition, while an electromagnetic striker pin operates atcurrent levels reached in short-circuit conditions, orv An electronic relay operated rom current transormers, one o which is installed oneach phase

b A space allocated to the several types o terminal currently used or the mainpower circuit conductors

Domestic circuit-breakers (see Fig. H25 next page) complying with IEC 60898 andsimilar national standards perorm the basic unctions o:

b Isolation

b

Protection against overcurrent

Power circuit terminals

Trip mechanism andprotective devices

Latching mechanism

Contacts and arc-divingchamber

Fool-proof mechanicalindicator

Fig. H24 : Main parts o a circuit-breaker

Industrial circuit-breakers must comply with

IEC 60947-1 and 60947-2 or other equivalent standards.

Domestic-type circuit-breakers must comply with

IEC standard 60898, or an equivalent national standard

Functions Possible conditions

Isolation b

Control Functional b

Emergency switching b (With the possibility o a trippingcoil or remote control)

Switching-o or mechanical b

maintenance

Protection Overload b

Short-circuit b

Insulation ault b (With dierential-current relay)

Undervoltage b (With undervoltage-trip coil)

Remote control b Added or incorporated

Indication and measurement b (Generally optional with anelectronic tripping device)


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Some models can be adapted to provide sensitive detection (30 mA) o earth-leakage current with CB tripping, by the addition o a modular block, while other

models (RCBOs, complying with IEC 61009 and CBRs complying with IEC 60947-2Annex B) have this residual current eature incorporated as shown in Figure H26.

Apart rom the above-mentioned unctions urther eatures can be associated withthe basic circuit-breaker by means o additional modules, as shown in Figure H27;notably remote control and indication (on-o-ault).

O - O F F

O - O F F

O - O F F

-

-

1

2

3

4

5

Fig. H27 : “Multi 9” system o LV modular switchgear components

Fig. H29 : Example o air circuit-breakers. Masterpact provides many control eatures in its “Micrologic” tripping unit

Moulded-case circuit-breakers complying with IEC 60947-2 are available rom 100to 630 A and provide a similar range o auxiliary unctions to those described above(see Figure H28).

Air circuit-breakers o large current ratings, complying with IEC 60947-2, aregenerally used in the main switch board and provide protector or currents rom630 A to 6300 A, typically.(see Figure H29).

In addition to the protection unctions, the Micrologic unit provides optimizedunctions such as measurement (including power quality unctions), diagnosis,communication, control and monitoring.

Fig. H25 : Domestic-type circuit-breaker providing overcurrent protection and circuit isolation eatures

Fig. H26 : Domestic-type circuit-breaker as above (Fig. H25)

with incorparated protection against electric shocks

Fig. H28 : Example o a Compact NSX industrial type o circuit- breaker capable o numerous auxiliary unctions

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4 Circuit-breaker

4.2 Fundamental characteristics o a circuit-breaker

The undamental characteristics o a circuit-breaker are:b Its rated voltage Ue

b Its rated current In

b Its tripping-current-level adjustment ranges or overload protection (Ir(1) or Irth(1))and or short-circuit protection (Im)(1)

b Its short-circuit current breaking rating (Icu or industrial CBs; Icn or domestic-type CBs).

Rated operational voltage (Ue)

This is the voltage at which the circuit-breaker has been designed to operate, innormal (undisturbed) conditions.

Other values o voltage are also assigned to the circuit-breaker, corresponding todisturbed conditions, as noted in sub-clause 4.3.

Rated current (In)

This is the maximum value o current that a circuit-breaker, tted with a speciedovercurrent tripping relay, can carry indenitely at an ambient temperature stated bythe manuacturer, without exceeding the specied temperature limits o the currentcarrying parts.

Example

A circuit-breaker rated at In = 125 A or an ambient temperature o 40 °C will beequipped with a suitably calibrated overcurrent tripping relay (set at 125 A). Thesame circuit-breaker can be used at higher values o ambient temperature however,i suitably “derated”. Thus, the circuit-breaker in an ambient temperature o 50 °Ccould carry only 117 A indenitely, or again, only 109 A at 60 °C, while complyingwith the specied temperature limit.

Derating a circuit-breaker is achieved thereore, by reducing the trip-current settingo its overload relay, and marking the CB accordingly. The use o an electronic-typeo tripping unit, designed to withstand high temperatures, allows circuit-breakers(derated as described) to operate at 60 °C (or even at 70 °C) ambient.

Note: In or circuit-breakers (in IEC 60947-2) is equal to Iu or switchgear generally,Iu being the rated uninterrupted current.

Frame-size rating

A circuit-breaker which can be tted with overcurrent tripping units o dierent currentlevel-setting ranges, is assigned a rating which corresponds to the highest current-level-setting tripping unit that can be tted.

Example

A Compact NSX630N circuit-breaker can be equipped with 11 electronic trip unitsrom 150 A to 630 A. The size o the circuit-breaker is 630 A.

Overload relay trip-current setting (Irth or Ir)

Apart rom small circuit-breakers which are very easily replaced, industrial circuit-breakers are equipped with removable, i.e. exchangeable, overcurrent-trip relays.Moreover, in order to adapt a circuit-breaker to the requirements o the circuitit controls, and to avoid the need to install over-sized cables, the trip relays are

generally adjustable. The trip-current setting Ir or Irth (both designations arein common use) is the current above which the circuit-breaker will trip. It alsorepresents the maximum current that the circuit-breaker can carry without tripping.That value must be greater than the maximum load current IB, but less than themaximum current permitted in the circuit Iz (see chapter G, sub-clause 1.3).

The thermal-trip relays are generally adjustable rom 0.7 to 1.0 times In, but whenelectronic devices are used or this duty, the adjustment range is greater; typically 0.4to 1 times In.

Example (see Fig. H30)

A NSX630N circuit-breaker equipped with a 400 A Micrologic 6.3E overcurrent triprelay, set at 0.9, will have a trip-current setting:

Ir = 400 x 0.9 = 360 A

Note: For circuit-breakers equipped with non-adjustable overcurrent-trip relays,

Ir = In. Example: or C60N 20 A circuit-breaker, Ir = In = 20 A.

(1) Current-level setting values which reer to the current-

operated thermal and “instantaneous” magnetic trippingdevices or over-load and short-circuit protection.

0.4 In

160 A 360 A 400 A 630 A

Rated current ofthe tripping unit

In

Overload trip

current setting

Ir

Adjustmentrange

Circuit breaker

frame-size rating

Fig. H30 : Example of a NSX630N circuit-breaker equipped with

a Micrologic 6.3E trip unit adjusted to 0.9, to give I r = 360 A

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Short-circuit relay trip-current setting (Im)

Short-circuit tripping relays (instantaneous or slightly time-delayed) are intended totrip the circuit-breaker rapidly on the occurrence o high values o ault current. Theirtripping threshold Im is:

b Either xed by standards or domestic type CBs, e.g. IEC 60898, or,

b Indicated by the manuacturer or industrial type CBs according to relatedstandards, notably IEC 60947-2.

For the latter circuit-breakers there exists a wide variety o tripping devices whichallow a user to adapt the protective perormance o the circuit-breaker to theparticular requirements o a load (see Fig. H3, Fig. H32 and Fig. H33).

Fig. H31 : Tripping-current ranges o overload and short-circuit protective devices or LV circuit-breakers

I(AIm

t (s )

Ir IcuIi

Fig. H33 : Perormance curve o a circuit-breaker electronic protective scheme

Ir: Overload (thermal or long-delay) relay trip-currentsetting

Im: Short-circuit (magnetic or shor t-delay) relay trip-current setting

Ii: Short-circuit instantaneous relay trip-current setting.

Icu: Breaking capacity

I(A

Im

t (s )

Ir Icu

Fig. H32 : Perormance curve o a circuit-breaker thermal-

magnetic protective scheme

(1) 50 In in IEC 60898, which is considered to be unrealistically high by most European manuacturers (Merlin Gerin = 10 to 14 In).

(2) For industrial use, IEC standards do not speciy values. The above values are given only as being those in common use.

Type o Overload Short-circuit protectionprotective protection

relay

Domestic Thermal- Ir = In Low setting Standard setting High setting circuit

breakers magnetic type B type C type DIEC 60898 3 In y Im y 5 In 5 In y Im y 10 In 10 In y Im y 20 In(1)

Modular Thermal- Ir = In Low setting Standard setting High settingindustrial(2) magnetic xed type B or Z type C type D or K

circuit-breakers 3.2 In y xed y 4.8 In 7 In y xed y 10 In 10 In y xed y 14 In

Industrial(2) Thermal- Ir = In xed Fixed: Im = 7 to 10 Incircuit-breakers magnetic Adjustable: Adjustable:

IEC 60947-2 0.7 In y Ir y In - Low setting : 2 to 5 In

- Standard setting: 5 to 10 In

Electronic Long delay Short-delay, adjustable

0.4 In y Ir y In 1.5 Ir y Im y 10 Ir

Instantaneous (I) xed

I = 12 to 15 In

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Isolating eature

A circuit-breaker is suitable or isolating a circuit i it ullls all the conditions

prescribed or a disconnector (at its rated voltage) in the relevant standard (seesub-clause 1.2). In such a case it is reerred to as a circuit-breaker-disconnector andmarked on its ront ace with the symbolAll Multi 9, Compact NSX and Masterpact LV switchgear o Schneider Electricranges are in this category.

Rated short-circuit breaking capacity (Icu or Icn)

The short-circuit current-breaking rating o a CB is the highest (prospective) valueo current that the CB is capable o breaking without being damaged. The valueo current quoted in the standards is the rms value o the AC component o theault current, i.e. the DC transient component (which is always present in the worstpossible case o short-circuit) is assumed to be zero or calculating the standardizedvalue. This rated value (Icu) or industrial CBs and (Icn) or domestic-type CBs isnormally given in kA rms.

Icu (rated ultimate s.c. breaking capacity) and Ics (rated service s.c. breaking

capacity) are dened in IEC 60947-2 together with a table relatingIcs with

Icu ordierent categories o utilization A (instantaneous tripping) and B (time-delayed

tripping) as discussed in subclause 4.3.

Tests or proving the rated s.c. breaking capacities o CBs are governed bystandards, and include:

b Operating sequences, comprising a succession o operations, i.e. closing andopening on short-circuit

b Current and voltage phase displacement. When the current is in phase with thesupply voltage (cos ϕ or the circuit = 1), interruption o the current is easier thanthat at any other power actor. Breaking a current at low lagging values o cos ϕ isconsiderably more dicult to achieve; a zero power-actor circuit being (theoretically)the most onerous case.

In practice, all power-system short-circuit ault currents are (more or less) at laggingpower actors, and standards are based on values commonly considered to berepresentative o the majority o power systems. In general, the greater the level oault current (at a given voltage), the lower the power actor o the ault-current loop,

or example, close to generators or large transormers.Figure H34 below extracted rom IEC 60947-2 relates standardized values o cos ϕ to industrial circuit-breakers according to their rated Icu.b Following an open - time delay - close/open sequence to test the Icu capacity o aCB, urther tests are made to ensure that:

v The dielectric withstand capabilityv The disconnection (isolation) perormance and

v The correct operation o the overload protection

have not been impaired by the test.

4.3 Other characteristics o a circuit-breaker

Rated insulation voltage (Ui)

This is the value o voltage to which the dielectric tests voltage (generally greaterthan 2 Ui) and creepage distances are reerred to.

The maximum value o rated operational voltage must never exceed that o the ratedinsulation voltage, i.e. Ue y Ui.

Familiarity with the ollowing characteristics o

LV circuit-breakers is oten necessary when making a fnal choice.

4 Circuit-breaker

The short-circuit current-breaking perormance o a LV circuit-breaker is related (approximately)

to the cos ϕ o the ault-current loop. Standard values or this relationship have been established in some standards

Icu cos ϕ

6 kA < Icu y 10 kA 0.5

10 kA < Icu y 20 kA 0.3

20 kA < Icu y 50 kA 0.25

50 kA < Icu 0.2

Fig. H34 : I cu related to power actor (cos ϕ ) o ault-current circuit (IEC 60947-2)

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4 Circuit-breaker

Fault-current limitation

The ault-current limitation capacity o a CB concerns its ability, more or less

eective, in preventing the passage o the maximum prospective ault-current,permitting only a limited amount o current to fow, as shown in Figure H38.The current-limitation perormance is given by the CB manuacturer in the orm ocurves (see Fig. H39).

b Diagram (a) shows the limited peak value o current plotted against the rmsvalue o the AC component o the prospective ault current (“prospective” ault-current reers to the ault-current which would fow i the CB had no current-limitingcapability)

b Limitation o the current greatly reduces the thermal stresses (proportional I2t) andthis is shown by the curve o diagram (b) o Figure H39, again, versus the rms valueo the AC component o the prospective ault current.

LV circuit-breakers or domestic and similar installations are classied in certainstandards (notably European Standard EN 60 898). CBs belonging to one class (ocurrent limiters) have standardized limiting I2t let-through characteristics dened bythat class.

In these cases, manuacturers do not normally provide characteristic perormancecurves.

Many designs o LV circuit-breakers eature

a short-circuit current limitation capability,whereby the current is reduced and prevented

rom reaching its (otherwise) maximum peak value (see Fig. H38). The current-limitation perormance o these CBs is presented in

the orm o graphs, typifed by that shown in Figure H39, diagram (a)

150 kA

Limitedcurrent peak(A2 x s)

2.105

4,5.105

Prospective ACcomponent (rms)

a)

Limitedcurrentpeak(kA)

N o n

- l i m i t e

d c u

r r e n t

c h a r a c t e

r i s t i c

s

150 kA

22

Prospective ACcomponent (rms)

b)

Fig. H39 : Perormance curves o a typical LV current-limiting circuit-breaker

Current limitation reduces both thermal and electrodynamic stresses on all circuit elements

through which the current passes, thereby prolonging the useul lie o these elements.

Furthermore, the limitation eature allows “cascading” techniques to be used (see 4.5)

thereby signifcantly reducing design and

installation costs

The advantages o current limitation

The use o current-limiting CBs aords numerous advantages:

b Better conservation o installation networks: current-limiting CBs strongly attenuateall harmul eects associated with short-circuit currents

b Reduction o thermal eects: Conductors (and thereore insulation) heating issignicantly reduced, so that the lie o cables is correspondingly increased

b Reduction o mechanical eects: orces due to electromagnetic repulsion are lower,with less risk o deormation and possible rupture, excessive burning o contacts, etc.

b Reduction o electromagnetic-intererence eects:

v Less infuence on measuring instruments and associated circuits,telecommunication systems, etc.

These circuit-breakers thereore contribute towards an improved exploitation o:

b Cables and wiring

b Preabricated cable-trunking systemsb Switchgear, thereby reducing the ageing o the installation

Example

On a system having a prospective shortcircuit current o 150 kA rms, a Compact Lcircuit-breaker limits the peak current to less than 10% o the calculated prospectivepeak value, and the thermal eects to less than 1% o those calculated.

Cascading o the several levels o distribution in an installation, downstream o alimiting CB, will also result in important savings.

The technique o cascading, described in sub-clause 4.5 allows, in act, substantialsavings on switchgear (lower perormance permissible downstream o the limitingCB(s)) enclosures, and design studies, o up to 20% (overall).

Discriminative protection schemes and cascading are compatible, in the CompactNSX range, up to the ull short-circuit breaking capacity o the switchgear.Fig. H38 : Prospective and actual currents

Icc

t

Limitedcurrent peak

Limitedcurrent

tc

Prospecticefault-current

Prospecticefault-current peak

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4.4 Selection o a circuit-breaker

Choice o a circuit-breakerThe choice o a CB is made in terms o:

b Electrical characteristics o the installation or which the CB is intended

b Its eventual environment: ambient temperature, in a kiosk or switchboardenclosure, climatic conditions, etc.

b Short-circuit current breaking and making requirements

b Operational specications: discriminative tripping, requirements (or not) orremote control and indication and related auxiliary contacts, auxiliary tripping coils,connection

b Installation regulations; in particular: protection o persons

b Load characteristics, such as motors, fuorescent lighting, LV/LV transormers

The ollowing notes relate to the choice LV circuit-breaker or use in distributionsystems.

Choice o rated current in terms o ambient temperature

The rated current o a circuit-breaker is dened or operation at a given ambienttemperature, in general:

b 30 °C or domestic-type CBs

b 40 °C or industrial-type CBs

Perormance o these CBs in a dierent ambient temperature depends mainly on thetechnology o their tripping units (see Fig. H40).

The choice o a range o circuit-breakers is determined by: the electrical characteristics o

the installation, the environment, the loads and a need or remote control, together with the type o telecommunications system envisaged

Ambienttemperature

Single CBin free air

Circuit breakers installedin an enclosure

Ambienttemperature

Temperature of ai rsurrouding the

circuit breakers

Fig. H40 : Ambient temperature

Circuit-breakers with uncompensated thermal

tripping units have a trip current level that depends on the surrounding temperature

Uncompensated thermal magnetic tripping units

Circuit-breakers with uncompensated thermal tripping elements have a tripping-current level that depends on the surrounding temperature. I the CB is installedin an enclosure, or in a hot location (boiler room, etc.), the current required to tripthe CB on overload will be sensibly reduced. When the temperature in which theCB is located exceeds its reerence temperature, it will thereore be “derated”. Forthis reason, CB manuacturers provide tables which indicate actors to apply at

temperatures dierent to the CB reerence temperature. It may be noted rom typicalexamples o such tables (see Fig. H4) that a lower temperature than the reerencevalue produces an up-rating o the CB. Moreover, small modular-type CBs mountedin juxtaposition, as shown typically in Figure H27, are usually mounted in a smallclosed metal case. In this situation, mutual heating, when passing normal loadcurrents, generally requires them to be derated by a actor o 0.8.

Example

What rating (In) should be selected or a C60 N?

b Protecting a circuit, the maximum load current o which is estimated to be 34 A

b Installed side-by-side with other CBs in a closed distribution box

b In an ambient temperature o 50 °C

A C60N circuit-breaker rated at 40 A would be derated to 35.6 A in ambient air at50 °C (see Fig. H41). To allow or mutual heating in the enclosed space, however, the0.8 actor noted above must be employed, so that, 35.6 x 0.8 = 28.5 A, which is notsuitable or the 34 A load.

A 50 A circuit-breaker would thereore be selected, giving a (derated) current ratingo 44 x 0.8 = 35.2 A.

Compensated thermal-magnetic tripping units

These tripping units include a bi-metal compensating strip which allows the overloadtrip-current setting (Ir or Irth) to be adjusted, within a specied range, irrespective othe ambient temperature.

For example:

b In certain countries, the TT system is standard on LV distribution systems, anddomestic (and similar) installations are protected at the service position by a circuit-breaker provided by the supply authority. This CB, besides aording protectionagainst indirect-contact hazard, will trip on overload; in this case, i the consumerexceeds the current level stated in his supply contract with the power authority. Thecircuit-breaker (y 60 A) is compensated or a temperature range o - 5 °C to + 40 °C.b LV circuit-breakers at ratings y 630 A are commonly equipped with compensatedtripping units or this range (- 5 °C to + 40 °C)

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4 Circuit-breaker

Electronic trip units

An important advantage with electronic tripping units is their stable perormancein changing temperature conditions. However, the switchgear itsel oten imposesoperational limits in elevated temperatures, so that manuacturers generally providean operating chart relating the maximum values o permissible trip-current levels tothe ambient temperature (see Fig. H42).

Moreover, electronic trip units can provide inormation that can be used or a bettermanagement o the electrical distribution, including energy eciency and powerquality.

C60a, C60H: curve C. C60N: curves B and C (reerence temperature: 30 °C)

Rating (A) 20 °C 25 °C 30 °C 35 °C 40 °C 45 °C 50 °C 55 °C 60 °C

1 1.05 1.02 1.00 0.98 0.95 0.93 0.90 0.88 0.852 2.08 2.04 2.00 1.96 1.92 1.88 1.84 1.80 1.74

3 3.18 3.09 3.00 2.91 2.82 2.70 2.61 2.49 2.37

4 4.24 4.12 4.00 3.88 3.76 3.64 3.52 3.36 3.24

6 6.24 6.12 6.00 5.88 5.76 5.64 5.52 5.40 5.30

10 10.6 10.3 10.0 9.70 9.30 9.00 8.60 8.20 7.80

16 16.8 16.5 16.0 15.5 15.2 14.7 14.2 13.8 13.5

20 21.0 20.6 20.0 19.4 19.0 18.4 17.8 17.4 16.8

25 26.2 25.7 25.0 24.2 23.7 23.0 22.2 21.5 20.7

32 33.5 32.9 32.0 31.4 30.4 29.8 28.4 28.2 27.5

40 42.0 41.2 40.0 38.8 38.0 36.8 35.6 34.4 33.2

50 52.5 51.5 50.0 48.5 47.4 45.5 44.0 42.5 40.5

63 66.2 64.9 63.0 61.1 58.0 56.7 54.2 51.7 49.2

Fig. H41 : Examples o tables or the determination o derating/uprating actors to apply to CBs

with uncompensated thermal tripping units, according to temperature

Electronic tripping units are highly stable in changing temperature levels

In (A)Coeff.

2,0001

NW20 withdrawable withhorizontal plugs

NW20 L1 withdrawablewith on edge plugs

1,8900.95

1,8000.90

20 50 55 6035 40 4525 30θ°C

Fig. H42 : Derating o Masterpact NW20 circuit-breaker, according to the temperature

Masterpact NW20 version 40°C 45°C 50°C 55°C 60°C

H1/H2/H3 Withdrawable with In (A) 2,000 2,000 2,000 1,980 1,890

horizontal plugs Maximum 1 1 1 0.99 0.95adjustment Ir

L1 Withdrawable with In (A) 2,000 200 1,900 1,850 1,800

on-edge plugs Maximum 1 1 0.95 0.93 0.90

adjustment Ir

Compact NSX00-250 N/H/L equippment with TM-D or TM-G trip units

Rating Temperature (°C)

(A) 0 5 20 25 30 35 40 45 50 55 60 65 70

16 18.4 18.7 18 18 17 16.6 16 15.6 15.2 14.8 14.5 14 13.825 28.8 28 27.5 25 26.3 25.6 25 24.5 24 23.5 23 22 21

32 36.8 36 35.2 34.4 33.6 32.8 32 31.3 30.5 30 29.5 29 28.5

40 46 45 44 43 42 41 40 39 38 37 36 35 34

50 57.5 56 55 54 52.5 51 50 49 48 47 46 45 44

63 72 71 69 68 66 65 63 61.5 60 58 57 55 54

80 92 90 88 86 84 82 80 78 76 74 72 70 68

100 115 113 110 108 105 103 100 97.5 95 92.5 90 87.5 85

125 144 141 138 134 131 128 125 122 119 116 113 109 106

160 184 180 176 172 168 164 160 156 152 148 144 140 136

200 230 225 220 215 210 205 200 195 190 185 180 175 170

250 288 281 277 269 263 256 250 244 238 231 225 219 213

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Selection o an instantaneous, or short-time-delay, trippingthreshold

Figure H43 below summarizes the main characteristics o the instantaneous orshort-time delay trip units.

Selection o a circuit-breaker according to the short-circuitbreaking capacity requirements

The installation o a circuit-breaker in a LV installation must ull one o the twoollowing conditions:b Either have a rated short-circuit breaking capacity Icu (or Icn) which is equal to orexceeds the prospective short-circuit current calculated or its point o installation, or

b I this is not the case, be associated with another device which is locatedupstream, and which has the required short-circuit breaking capacity

In the second case, the characteristics o the two devices must be co-ordinatedsuch that the energy permitted to pass through the upstream device must notexceed that which the downstream device and all associated cables, wires and othercomponents can withstand, without being damaged in any way. This technique isprotably employed in:

b Associations o uses and circuit-breakersb Associations o current-limiting circuit-breakers and standard circuit-breakers.The technique is known as “cascading” (see sub-clause 4.5 o this chapter)

The selection o main and principal circuit-breakers

A single transormer

I the transormer is located in a consumer’s substation, certain national standardsrequire a LV circuit-breaker in which the open contacts are clearly visible such asCompact NSX withdrawable circuit-breaker.

Example (see Fig. H44 opposite page)

What type o circuit-breaker is suitable or the main circuit-breaker o an installationsupplied through a 250 kVA MV/LV (400 V) 3-phase transormer in a consumer’ssubstation?

In transormer = 360 A

Isc (3-phase) = 8.9 kA

A Compact NSX400N with an adjustable tripping-unit range o 160 A - 400 A and ashort-circuit breaking capacity (Icu) o 50 kA would be a suitable choice or this duty.

The installation o a LV circuit-breaker requires that its short-circuit breaking capacity (or that o the CB together with an associated device) be

equal to or exceeds the calculated prospective short-circuit current at its point o installation

The circuit-breaker at the output o the smallest

transormer must have a short-circuit capacity adequate or a ault current which is higher than that through any o the other transormer

LV circuit-breakers

Fig. H43 : Dierent tripping units, instantaneous or short-time-delayed

Type Tripping unit Applications

Low setting b Sources producing low short-circuit-

type B current levels(standby generators)

b Long lengths o line or cable

Standard setting b Protection o circuits: general case

type C

High setting b Protection o circuits having high initial

type D or K transient current levels(e.g. motors, transormers, resistive loads)

12 In b Protection o motors in association withtype MA discontactors

(contactors with overload protection)

I

t

I

t

I

t

I

t

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Several transormers in parallel (see Fig. H45)

b The circuit-breakers CBP outgoing rom the LV distribution board must each be

capable o breaking the total ault current rom all transormers connected to thebusbars, viz: Isc1 + Isc2 + Isc3

b The circuit-breakers CBM, each controlling the output o a transormer, must becapable o dealing with a maximum short-circuit current o (or example) Isc2 + Isc3only, or a short-circuit located on the upstream side o CBM1.

From these considerations, it will be seen that the circuit-breaker o the smallesttransormer will be subjected to the highest level o ault current in thesecircumstances, while the circuit-breaker o the largest transormer will pass thelowest level o short-circuit current

b The ratings o CBMs must be chosen according to the kVA ratings o theassociated transormers

Note: The essential conditions or the successul operation o 3-phase transormersin parallel may be summarized as ollows:

1. the phase shit o the voltages, primary to secondary, must be the same in all unitsto be paralleled.

2. the open-circuit voltage ratios, primary to secondary, must be the same in all units.

3. the short-circuit impedance voltage (Zsc%) must be the same or all units.For example, a 750 kVA transormer with a Zsc = 6% will share the load correctlywith a 1,000 kVA transormer having a Zsc o 6%, i.e. the transormers will be loadedautomatically in proportion to their kVA ratings. For transormers having a ratio o kVAratings exceeding 2, parallel operation is not recommended.

Figure H46 indicates, or the most usual arrangement (2 or 3 transormers oequal kVA ratings) the maximum short-circuit currents to which main and principalCBs (CBM and CBP respectively, in Figure H45) are subjected. It is based on theollowing hypotheses:

b The short-circuit 3-phase power on the MV side o the transormer is 500 MVA

b The transormers are standard 20/0.4 kV distribution-type units rated as listed

b The cables rom each transormer to its LV circuit-breaker comprise 5 metres osingle core conductors

b Between each incoming-circuit CBM and each outgoing-circuit CBP there is1 metre o busbar

b The switchgear is installed in a foormounted enclosed switchboard, in an ambient-air temperature o 30 °C

Moreover, this table shows selected circuit-breakers o M-G manuacturerecommended or main and principal circuit-breakers in each case.

Example (see Fig. H47 next page)

b Circuit-breaker selection or CBM duty:For a 800 kVA transormer In = 1.126 A; Icu (minimum) = 38 kA (rom Figure H46),the CBM indicated in the table is a Compact NS1250N (Icu = 50 kA)

b Circuit-breaker selection or CBP duty:The s.c. breaking capacity (Icu) required or these circuit-breakers is given in theFigure H46 as 56 kA.A recommended choice or the three outgoing circuits 1, 2 and 3 would be current-limiting circuit-breakers types NSX400 L, NSX250 L and NSX100 L. The Icu rating ineach case = 150 kA.

Compact

NSX400N

250 kVA

20 kV/400 V

MV

Tr1

LV

CBMA1

B1

CBP

MV

Tr2

LV

CBMA2

B2

CBP

MV

Tr3

LV

CBMA3

B3

E

Fig. H44 : Example o a transormer in a consumer’s substation

Fig. H45 : Transormers in parallel

4 Circuit-breaker

Fig. H46 : Maximum values of short-circuit current to be interrupted by main and principal circuit-breakers (CBM and CBP respectively), for several transformers in parallel

Number and kVA ratings Minimum S.C breaking Main circuit-breakers (CBM) Minimum S.C breaking Rated current In oo 20/0.4 kV transormers capacity o main CBs total discrimination with out capacity o principal CBs principal circuit-breaker

(Icu) kA going circuit-breakers (CBP) (Icu) kA (CPB) 250A

2 x 400 14 NW08N1/NS800N 27 NSX250H

3 x 400 28 NW08N1/NS800N 42 NSX250H

2 x 630 22 NW10N1/NS1000N 42 NSX250H

3 x 630 44 NW10N1/NS1000N 67 NSX250H

2 x 800 19 NW12N1/NS1250N 38 NSX250H

3 x 800 38 NW12N1/NS1250N 56 NSX250H

2 x 1,000 23 NW16N1/NS1600N 47 NSX250H

3 x 1,000 47 NW16N1/NS1600N 70 NSX250H

2 x 1,250 29 NW20N1/NS2000N 59 NSX250H

3 x 1,250 59 NW20N1/NS2000N 88 NSX250L

2 x 1,600 38 NW25N1/NS2500N 75 NSX250L

3 x 1,600 75 NW25N1/NS2500N 113 NSX250L

2 x 2,000 47 NW32N1/NS3200N 94 NSX250L3 x 2,000 94 NW32N1/NS3200N 141 NSX250L

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These circuit-breakers provide the advantages o:v Absolute discrimination with the upstream (CBM) breakers

v Exploitation o the “cascading” technique, with its associated savings or alldownstream components

Choice o outgoing-circuit CBs and fnal-circuit CBs

Use o table G40

From this table, the value o 3-phase short-circuit current can be determined rapidlyor any point in the installation, knowing:

b The value o short-circuit current at a point upstream o that intended or the CBconcerned

b The length, c.s.a., and the composition o the conductors between the two points

A circuit-breaker rated or a short-circuit breaking capacity exceeding the tabulatedvalue may then be selected.

Detailed calculation o the short-circuit current level

In order to calculate more precisely the short-circuit current, notably, when the short-circuit current-breaking capacity o a CB is slightly less than that derived rom the

table, it is necessary to use the method indicated in chapter G clause 4.

Two-pole circuit-breakers (or phase and neutral) with one protected pole only

These CBs are generally provided with an overcurrent protective device on thephase pole only, and may be used in TT, TN-S and IT schemes. In an IT scheme,however, the ollowing conditions must be respected:

b Condition (B) o table G67 or the protection o the neutral conductor againstovercurrent in the case o a double ault

b Short-circuit current-breaking rating: A 2-pole phase-neutral CB must, byconvention, be capable o breaking on one pole (at the phase-to-phase voltage) thecurrent o a double ault equal to 15% o the 3-phase short-circuit current at the pointo its installation, i that current is y 10 kA; or 25% o the 3-phase short-circuit currenti it exceeds 10 kA

b Protection against indirect contact: this protection is provided according to therules or IT schemes

Insufcient short-circuit current breaking rating

In low-voltage distribution systems it sometimes happens, especially in heavy-dutynetworks, that the Isc calculated exceeds the Icu rating o the CBs available orinstallation, or system changes upstream result in lower level CB ratings beingexceeded

b Solution 1: Check whether or not appropriate CBs upstream o the CBs aectedare o the current-limiting type, allowing the principle o cascading (described in sub-clause 4.5) to be applied

b Solution 2: Install a range o CBs having a higher rating. This solution iseconomically interesting only where one or two CBs are aected

b Solution 3: Associate current-limiting uses (gG or aM) with the CBs concerned, onthe upstream side. This arrangement must, however, respect the ollowing rules:v The use rating must be appropriatev No use in the neutral conductor, except in certain IT installations where a doubleault produces a current in the neutral which exceeds the short-circuit breaking ratingo the CB. In this case, the blowing o the neutral use must cause the CB to trip onall phases

4.5 Coordination between circuit-breakers

Cascading

Defnition o the cascading technique

By limiting the peak value o short-circuit current passing through it, a current-limitingCB permits the use, in all circuits downstream o its location, o switchgear andcircuit components having much lower short-circuit breaking capacities, and thermaland electromechanical withstand capabilities than would otherwise be necessary.Reduced physical size and lower perormance requirements lead to substantialeconomy and to the simplication o installation work. It may be noted that, while acurrent-limiting circuit-breaker has the eect on downstream circuits o (apparently)increasing the source impedance during short-circuit conditions, it has no sucheect in any other condition; or example, during the starting o a large motor (where

a low source impedance is highly desirable). The range o Compact NSX current-limiting circuit-breakers with powerul limiting perormances is particularly interesting.

Short-circuit ault-current levels at any point in

an installation may be obtained rom tables

The technique o “cascading” uses the properties o current-limiting circuit-breakers

to permit the installation o all downstream switchgear, cables and other circuit components

o signifcantly lower perormance than would otherwise be necessary, thereby simpliying and reducing the cost o an installation

CBP1

3 Tr

800 kVA

20 kV/400 V

CBM

400 A

CBP2

100 A

CBP3

200 A

Fig. H47 : Transormers in parallel

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4 Circuit-breaker

Conditions o implementation

Most national standards admit the cascading technique, on condition that the

amount o energy “let through” by the limiting CB is less than the energy alldownstream CBs and components are able to withstand without damage.

In practice this can only be veried or CBs by tests perormed in a laboratory. Suchtests are carried out by manuacturers who provide the inormation in the orm otables, so that users can condently design a cascading scheme based on thecombination o recommended circuit-breaker types. As an example, Figure H48 indicates the cascading possibilities o circuit-breaker types C60, DT40N, C120 andNG125 when installed downstream o current-limiting CBs Compact NSX 250 N, Hor L or a 230/400 V or 240/415 V 3-phase installation.

In general, laboratory tests are necessary to

ensure that the conditions o implementation

required by national standards are met and compatible switchgear combinations must be

provided by the manuacturer

Fig. H48 : Example o cascading possibilities on a 230/400 V or 240/415 V 3-phase installation

kA rms

Short-circuit 150 NSX250Lbreaking capacity 70 NSX250Ho the upstream 50 NSX250N(limiter) CBs

Possible short-circuit 150 NG125Lbreaking capacity o 70 NG125Lthe downstream CBs 36 NG125N NG125N(benefting rom the

30 C60N/H<=32A C60N/H<=32A C60N/H<=32Acascading technique)

30 C60L<=25A C60L<=25A(*) C60L<=25AQuick PRD

40/20/8

25 C60H>=40A C60H>=40A C60H>=40A

C120N/H C120N/H C120N/H

20 C60N>=40A C60N>=40A C60N>=40A

Advantages o cascadingThe current limitation benets all downstream circuits that are controlled by thecurrent-limiting CB concerned.

The principle is not restrictive, i.e. current-limiting CBs can be installed at any point inan installation where the downstream circuits would otherwise be inadequately rated.

The result is:

b Simplied short-circuit current calculations

b Simplication, i.e. a wider choice o downstream switchgear and appliances

b The use o lighter-duty switchgear and appliances, with consequently lower cost

b Economy o space requirements, since light-duty equipment have generally asmaller volume

Principles o discriminative tripping (selectivity)

Discrimination is achieved by automatic protective devices i a ault condition, occurringat any point in the installation, is cleared by the protective device located immediately upstream o the ault, while all other protective devices remain unaected (see

Fig. H49).

Isc

A

B

IscTotal discrimination

Isc BIr B

0

0Isc

Isc BIsIr B

Is = discrimination limit

B only opensPartial discrimination

A and B open

Fig. H49 : Total and partial discrimination

Discrimination may be total or partial, and based on the principles o current levels, or

time-delays, or a combination o both. A more

recent development is based on the logic techniques.

The Schneider Electric system takes advantages o both current-limitation and

discrimination

(*) Quick PRD with integrated circuit-breaker as disconnector see chapter J

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Discrimination between circuit-breakers A and B is total i the maximum value oshort-circuit-current on circuit B (Isc B) does not exceed the short-circuit trip setting

o circuit-breaker A (Im A). For this condition, B only will trip (see Fig. H50).

Discrimination is partial i the maximum possible short-circuit current on circuit Bexceeds the short-circuit trip-current setting o circuit-breaker A. For this maximumcondition, both A and B will trip (see Fig. H5).

Protection against overload : discrimination based on current levels(see Fig. H52a)

This method is realized by setting successive tripping thresholds at stepped levels,rom downstream relays (lower settings) towards the source (higher settings).Discrimination is total or par tial, depending on particular conditions, as noted above.As a rule o thumb, discrimination is achieved when:

b IrA/ IrB > 2:

Protection against low level short-circuit currents : discrimination based on

stepped time delays (see Fig. H52b)

This method is implemented by adjusting the time-delayed tripping units, such that

downstream relays have the shortest operating times, with progressively longer

delays towards the source.

In the two-level arrangement shown, upstream circuit-breaker A is delayed

suciently to ensure total discrimination with B (or example: Masterpact with

electronic trip unit).

Discrimination based on a combination o the two previous methods

(see Fig. H52c)

A time-delay added to a current level scheme can improve the overall discrimination

perormance.

The upstream CB has two high-speed magnetic tripping thresholds:

b Im A: delayed magnetic trip or short-delay electronic trip

b Ii: instantaneous strip

Discrimination is total i Isc B < Ii (instantaneous).

Protection against high level short-circuit currents: discrimination based on

arc-energy levels

This technology implemented in the Compact NSX range (current limiting circuit-

breaker) is extremely eective or achievement o total discrimination.

Principle: When a very high level short-circuit current is detected by the two circuits-

breaker A and B, their contacts open simultaneously. As a result, the current is highly

limited.

b The very high arc-energy at level B induces the tripping o circuit-breaker B

b Then, the arc-energy is limited at level A and is not sucient to induce the tripping

o A

As a rule o thumb, the discrimination between Compact NSX is total i the size ratio

between A and B is greater than 2.5.

t

Im A Is cBIr AIr B

B A

Is c A I

A and B openB only opens

Fig. H51 : Partial discrimination between CBs A and B

t

Im AIr AIr B

B A

Isc B

I

Fig. H50 : Total discrimination between CBs A and B

I

t

Ir AIr B

B A

a) b)

I

t

Isc B

∆t

A

B

A

B

c)

t

Im Adelayed

Isc B

I

B A

Ii Ainstantaneous

Fig. H52 : Discrimination

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4 Circuit-breaker

Discrimination quality Discrimination is total i I s > I sc(D2), i.e. I sd1 > I sc(D2).

This normally implies:

b a relatively low level I sc(D2),

b a large dierence between the ratings o circuit-breakers D1 and D2.

Current discrimination is normally used in fnal distribution.

Current-level discrimination

This technique is directly linked to the staging o the Long Time (LT) tripping curves

o two serial-connected circuit-breakers.

Discrimination based on time-delayed tripping

uses CBs reerred to as “selective” (in some countries).

Implementation o these CBs is relatively simple

and consists in delaying the instant o tripping o the several series-connected circuit-breakers in a stepped time sequence

Fig. H53 : Current discrimination

The discrimination limit ls is:

b Is = Isd2 i the thresholds lsd1 and lsd2 are too close or merge,

b Is = Isd1 i the thresholds lsd1 and lsd2 are suciently ar apart.

As a rule, current discrimination is achieved when:

b Ir1 / Ir2 < 2,

b Isd1 / Isd2 > 2.

The discrimination limit is:

b Is = Isd1.

Time discrimination

This is the extension o current discrimination and is obtained by staging over time

o the tripping curves. This technique consists o giving a time delay o t to the Short

Time (ST) tripping o D1.

Fig. H54 : Time discrimination

D1

D2

Isd 2 Isd1Ir2 Ir1

t D2 D1

I

D1

D2

Isd 2 Isd1 Ii1Ir2 Ir1

t

D2 D1

I

∆t

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The thresholds (Ir1, Isd1) o D1 and (Ir2, Isd2) comply with the staging rules o

current discrimination.

The discrimination limit ls o the association is at least equal to li1, the instantaneousthreshold o D1.

Discrimination quality

There are two possible applications:

b on fnal and/or intermediate eeders

A category circuit-breakers can be used with time-delayed tripping o the

upstream circuit-breaker. This allows extension o current discrimination up to the instantaneous threshold li1 o the upstream circuit-breaker: I s = li1.

I I sc(D2) is not too high - case o a fnal eeder - total discrimination can be obtained.

b on the incomers and eeders o the MSB

At this level, as continuity o supply takes priority, the installation

characteristics allow use o B category circuit-breakers designed or

time-delayed tripping. These circuit-breakers have a high thermal withstand ( I cw u 50% I cn or t = 1s): I s = I cw1.

Even or high lsc(D2), time discrimination normally provides total

discrimination: I cw1 > I cc(D2) .

Note: Use o B category circuit-breakers means that the installation must withstand

high electrodynamic and thermal stresses.

Consequently, these circuit-breakers have a high instantaneous threshold li that can

be adjusted and disabled in order to protect the busbars i necessary.

Practical example o discrimination at several levels with Schneider Electriccircuit-breakers (with electronic trip units)

"Masterpact NT is totally selective with any moulded-case Compact NSX circuitbreaker, i.e., the downstream circuit-breaker will trip or any short-circuit value up toits breaking capacity. Further, all Compact NSX CBs are totally selective, as long asthe ration between sizes is greater than 1.6 and the ratio between ratings is greater

than 2.5. The same rules apply or the total selectivity with the miniature circuit-breakers Multi9 urther downstream (see Fig. H55).

Fig. H55 : 4 level discrimination with Schneider Electric circuit breakers : Masterpact NT

Compact NSX and Multi 9

I

t

Icc B

A

Icc

B

Only B opens

Current-breakingtime for B

Non trippingtime of A

Ir B

Masterpact NT06630 A

Compact NSX250 A

Compact NSX100 A

Multi 9C60

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Energy discrimination with current limitation

Cascading between 2 devices is normally achieved by using the tripping o theupstream circuit-breaker A to help the downstream circuit-breaker B to break thecurrent. The discrimination limit Is is consequently equal to the ultimate breakingcurrent Icu B o circuit-breaker B acting alone, as cascading requires the tripping oboth devices.The energy discrimination technology implemented in Compact NSX circuit-breakersallows to improve the discrimination limit to a value higher than the ultimate breakingcurrent Icu B o the downstream circuit-breaker. The principle is as ollows:

b The downstream limiting circuit-breaker B sees a very high short-circuit current.The tripping is very ast (<1 ms) and then, the current is limited

b The upstream circuit-breaker A sees a limited short-circuit current compared to itsbreaking capability, but this current induces a repulsion o the contacts. As a result,the arcing voltage increases the current limitation. However, the arc energy is nothigh enough to induce the tripping o the circuit-breaker. So, the circuit-breaker Ahelps the circuit-breaker B to trip, without tripping itsel. The discrimination limit canbe higher than Icu B and the discrimination becomes total with a reduced cost o thedevices

Natural total discriminitation with Compact NSX

The major advantage o the Compact NSX range is to provide a natural totaldiscrimination between two series-connected devices i:

b The ratio o the two trip-unit current ratings is > 1.6

b The ratio o rated currents o the two circuit-breakers is > 2.5

Logic discrimination or “Zone Sequence Interlocking – ZSI”

This type o discrimination can be achieved with circuit-breakers equipped withspecially designed electronic trip units (Compact, Masterpact): only the Short TimeProtection (STP) and Ground Fault Protection (GFP) unctions o the controlleddevices are managed by Logic Discrimination. In particular, the InstantaneousProtection unction - inherent protection unction - is not concerned.

Settings o controlled circuit-breakers

b time delay: there are no rules, but staging (i any)o the time delays o time

discrimination must be applied (ΔtD1 u ΔtD2 u ΔtD3),

b thresholds: there are no threshold rules to be applied, but natural staging o theprotection device ratings must be complied with (IcrD1 u IcrD2 u IcrD3).

Note: This technique ensures discrimination even with circuit-breakers o similarratings.

Principles

Activation o the Logic Discrimination unction is via transmission o inormation onthe pilot wire:

b ZSI input:

v low level (no downstream aults): the Protection unction is on standby with areduced time delay (y 0,1 s),

v high level (presence o downstream aults): the relevant Protection unction movesto the time delay status set on the device.

b ZSI output:

v low level: the trip unit detects no aults and sends no orders,

v high level: the trip unit detects a ault and sends an order.

Operation

A pilot wire connects in cascading orm the protection devices o an installation(see Fig. H56). When a ault occurs, each circuit-breaker upstream o the ault(detecting a ault) sends an order (high level output) and moves the upstream circuit-breaker to its natural time delay (high level input). The circuitbreaker placed justabove the ault does not receive any orders (low level input) and thus trips almostinstantaneously.

Discrimination schemes based on logic

techniques are possible, using CBs equipped with electronic tripping units designed or

the purpose (Compact, Masterpact) and interconnected with pilot wires

Fig. H56 : Logic discrimination.

pilot wire

interlockingorder

interlockingorder

D1

D2

D3

4 Circuit-breaker

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d e r E l e c t r i c - a l l r i g h t s r e s e r v e d

4 Circuit-breaker

4.6 Discrimination MV/LV in a consumer’ssubstation

In general the transormer in a consumer’s substation is protected by MV uses,suitably rated to match the transormer, in accordance with the principles laid downin IEC 60787 and IEC 60420, by ollowing the advice o the use manuacturer.

The basic requirement is that a MV use will not operate or LV aults occurringdownstream o the transormer LV circuit-breaker, so that the tripping characteristiccurve o the latter must be to the let o that o the MV use pre-arcing curve.

This requirement generally xes the maximum settings or the LV circuit-breakerprotection:

b Maximum short-circuit current-level setting o the magnetic tripping element

b Maximum time-delay allowable or the short-circuit current tripping element(see Fig. H57)

Example:

b Short-circuit level at MV terminals o transormer: 250 MVA

b Transormer MV/LV: 1,250 kVA 20/0.4 kV

b MV uses: 63 A

b Cabling, transormer - LV circuit-breaker: 10 metres single-core cablesb LV circuit-breaker: Compact NSX 2000 set at 1,800 A (Ir)What is the maximum short-circuit trip current setting and its maximum time delayallowable?

The curves o Figure H58 show that discrimination is assured i the short-time delaytripping unit o the CB is set at:

b A level y 6 Ir = 10.8 kA

b A time-delay setting o step 1 or 2

Fig. H57 : Example

63 A

1,250 kVA

20 kV / 400 V

Compact

NS2000

set at 1,800 A

Full-load current

1,760 A

3-phase

short-circuit

current level

31.4 kA

I

t(s)

Step 4Step 3Step 2

Step 10.50

0.1

0.2

10

100

200

1,000

NS 2000

set at

1,800 A

1,800 A

Ir

Isc maxi31.4 kA

10 kA0.01

Minimum pre-arcing

curve or 63 A HV uses

(current reerred to the

secondary side o the

transormer)

1 4 6

8

Fig. H58 : Curves o MV uses and LV circuit-breaker

Discrimination quality

This technique enables:

b easy achievement as standard o discrimination on 3 levels or more,

b elimination o important stresses on the installation, relating to time- delayed tripping o the protection device, in event o a ault directly on the

upstream busbars.

All the protection devices are thus virtually instantaneous,

b easy achievement o downstream discrimination with non-controlled circuit-breakers.

H LV Switch Gear Functions and Selection

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